The present invention is related to image compression/decompression systems, and more particularly to a compression/decompression system for improving the perceived image quality of a video signal and the like, being transmitted from a wide bandwidth system to a narrow bandwidth that is, lower bit rate, system.
Video compression methods used on the Internet today use block transform coding similar to the coding scheme set by the Motion Picture Experts Group (MPEG). To this end, blocks of pixels from the representing image, or the difference from a previous image, are transformed. The transformed components are then rounded so as to require fewer bits to encode them. In the process of decoding, the blocks of rounded components are back transformed and used to reconstruct an approximation of the original image. To achieve the very high rates of compression required for the broadcast of video over the Internet, most of the transform components must be rounded to zero. The highest frequency components are zeroed out first. However, the result of setting non-zero transform components to zero does more than just reduce the image resolution, it also causes transition at the block edges in the decoded image. This blockiness becomes disturbing in Internet video.
The use of non-blocked encoding schemes such as sub-band coding avoid the blockiness but require more computer power to encode and decode the signals, precluding their use on the Internet. Another approach used on the Internet to reduce blockiness is to send frame updates less often. This results in ajerky video signal which also is objectionable.
Pre encoding filtering is provided to remove visually unimportant frequencies and particularly those frequencies that generate alias frequencies at the reduced size image that will be viewed on, for example, the Internet. Traditionally video filtering has been done with sharp cutoff filters that separate the pass band from the stop band. Such filters are used in digital television to remove components above half the sample frequency which cause unwanted alias frequencies in the video band. Flat pass bands are desired so that multiple passes of the video signal through equipment using such filters does not degrade the signal. However, sharp cutoff filters have a ringing impulse response. The resulting filter rings are detrimental to the encoding process since the encoder must encode both the feature and the ring associated with the feature, since the ring may not be in the same block as the feature. A ring associated with a feature is an artifact of filtering and reduces the perceived image quality.
Accordingly, it would be highly desirable to provide a video compression technique for use on the Internet, and other comparable transmission mediums, which minimizes the visual artifacts which presently are generated when video images are compressed for transmission through very narrow band channels, such as used on the Internet.
The present invention provides a method and associated apparatus for overcoming the shortcomings of the prior art compression schemes of previous mention presently in use for example on the Internet.
To this end, it is an object and associated advantage of the present invention to pre-filter the video image so as to improve the perceived image quality of, for example, Internet video signals when using typical Internet video coding/decoding (codec) systems.
It is another object and associated advantage of the present invention to provide the pre-filtering process prior to the processes of sizing and encoding the video signal, using a gaussian-like filter.
Another object and associated advantage of the invention is to provide the pre-encoding filtering with a two dimensional spatial impulse filter which has good pulse fidelity, rapid cutoff at high frequency and minimal impulse response width.
A still further object and associated advantage of the present invention is to provide a gaussian type pre-filter which removes alias frequencies in the video band without producing ringing.
A further object and associated advantage of the invention comprises removing spatial frequencies corresponding to the higher order discrete cosine transform components prior to the compression process to prevent the occurrence of blocky artifacts and loss of resolution which inherently are caused by setting the components to zero.
Accordingly, the present invention comprises a two dimensional spatial impulse filter with good pulse fidelity, rapid cutoff at high frequency and minimal impulse response width, which is inserted in the compression/transmission/decompression path of the video signal prior to the compression stage. Because the video only passes through the encoder once in, for example, an Internet transmission channel, filters with good pulse fidelity are used in the present invention. Such filters are similar to gaussian impulse response filters or raised cosine amplitude response filters. One dimensional spatial filters of this kind applied in both vertical and horizontal directions combine to make a two dimensional spatial impulse filter with circularly symmetric impulse response and frequency response. Thus the diagonal response is the same as the vertical and horizontal responses, which results in the best visual use of bandwidth.
By way of illustration of the invention, a raised cosine filter which is 6 db down at the cutoff frequency of a sharp cutoff filter actually has a narrower impulse response and almost no ringing. However, 6 db down at the spatial band edge (half the pixel frequency of the video to be coded) is not enough to remove all alias frequencies. It has been found that moving the 6 db point of a pulse filter to about 0.7 (0.6 to 0.9) of the spatial band edge removes visible aliases in the image while being a good compromise between subjective image sharpness and total picture entropy, that is, the amount of information in the picture.
The two dimensional spatial bandwidth resulting from applying sharp cutoff filters with spatial bandwidth k both horizontally and vertically is k{circumflex over ( )}2. The two dimensional spatial bandwidth resulting from applying an impulse filter with 0.7k bandwidth both horizontally and vertically is approximately (Pi/8) * k{circumflex over ( )}2. Thus for images with uniform spectral content the pre-filtering of the present invention reduces the amount of information to be encoded by about 60%. This allows video to be transmitted down narrow band, that is low bit rate, digital channels with visually improved results.
The reason for visual improvement is that for typical image sequences sent on very narrow band data channels, at least this much information will be removed by the encoder anyhow. When the encoder removes a large amount of information it introduces blockiness and loss of resolution which is more objectionable than the resolution loss introduced by the pre-filter. It follows therefore that the invention provides the visually improved results by preventing blockiness while removing the large amount of high frequency information to enhance the compression process.